What a removable mullion is
On a pair of doors, the mullion is the upright member in the center that the leaves close against. A fixed mullion is built into the frame permanently. A removable mullion does the same job, giving each leaf something to latch and lock into, but it can be released and lifted out when the whole opening needs to be clear.
That matters wherever large items move through a doorway, such as equipment rooms, loading areas, assembly halls and retail back-of-house. The rest of the time the mullion stays in place and the pair behaves like any other secured opening.
How they work
A removable mullion mounts into the frame at the top and bottom. The head and sill receive fittings that the mullion locks into, so it stands as a rigid post once installed. To remove it, you release it, usually by turning a key or operating a lever at the base, which disengages the top and bottom retainers and lets the mullion tilt and lift out.
Reinstalling is the reverse: set the bottom into its fitting, swing the top in, and lock it back down. The exit devices on the door leaves latch into or against the mullion, so with the mullion in place the pair is fully secured, and with it out the opening is wide open.
- Fittings at the head and sill receive and retain the mullion
- Keyed release is the common method; some use a lever at the base
- Exit devices on the leaves latch against the mullion when it is in
- Von Duprin is one well-known maker of removable mullions
Keyed and electrified options
A keyed removable mullion is released with a key, so only someone with the key can take it out. This is the most common type, keeping the clear-opening function controlled.
An electrified removable mullion carries wiring through the mullion so it can integrate with an access-control or electric-latch-retraction system. That lets the opening tie into card readers, remote release or monitoring, while still coming out when a clear opening is needed. Electrified versions add complexity: you have to plan how power gets to the mullion and how the wiring is handled each time it is removed.
Why they are used
The core reason is flexibility. A pair of doors with a mullion secures well and latches reliably, but the mullion narrows each half of the opening. A removable mullion gives you the security of a mullion most of the time and the full clear width when you need it, without committing to a permanently obstructed opening.
On exit and egress pairs this is especially useful, because panic hardware works cleanly against a mullion while the opening can still be fully cleared for moving large loads or for events.
Fire-rating considerations
On a fire-rated pair of doors, the mullion is often part of what makes the assembly carry its rating. A fire-exit pair may be required to have the mullion in place for the opening to meet its listing, so removing it is not simply a convenience, it can affect the rating.
The important point for estimators and installers is that the mullion, the doors, the frame and the exit devices are a listed assembly, and substitutions or removals have to respect that listing. When you price or submit a fire-rated opening with a removable mullion, keep the hardware set consistent with the listing and flag anything that departs from it.
Getting it right on the bid
Removable mullions show up on the hardware set for the opening, with the mullion itself, its stabilizer or retainers, and any electrification all specified. Catching that early keeps the quote and the order accurate. eMullion is built to keep those hardware sets straight across a whole project so a detail like a removable mullion does not slip between the schedule and the order.
